1. Field of the Invention
The invention relates to an automated machining process for machining of an obstructed passage of an article.
2. Brief Description of the Related Art
During the last decades the performance of gas turbine components has increased continuously. New, advanced materials have been introduced in order to comply with the high firing temperatures of modern engines. At the same time, component design has changed from not cooled, bulk components to hollow blades and vanes with a complex internal cooling structure. One of the latest developments is cooling channel configurations with a diffuser outlet, which typically has a fan shape geometry. In this design the diameter of the cylindrical section of the cooling channel defines the total airflow volume, whereas the fan shaped end of the channel results in an advantageous redistribution of the cooling air over the adjacent surface. According to the cooling requirements this film cooling effect can be adjusted by design variations of the diffuser outlet.
The latest generation of these gas turbine components has to withstand hot gas temperatures exceeding the melting point of highly alloyed, high-strength materials. In order to ensure safe and reliable operation of the engine, the designed functionality of the cooling system must be ensured during the whole lifetime of the hot gas path components. For this purpose protective coatings are applied to the outer (and sometimes also inner) surfaces of the parts.
In modern engines the number of cooling holes per component can be as high as several hundreds. The geometry of the cooling passages depends on their location on the component. It is not unusual that more than 20 different types of cooling holes can be found on a single component.
A typical manufacturing sequence of such a part starts with the casting. After the following machining a first metallic protection coating (usually of the MCrAlY family, where M is Ni and/or Co) is sprayed onto the component. A second, ceramic thermal barrier coating (TBC) is then deposited on the first coating. For most gas turbine components the thickness of either coating is in the range from 100 μm to 600 μm.
Cooling holes are only machined after both coatings have been deposited. The use of high power, pulsed lasers is common for this purpose. In most cases an initial start hole is created by percussion drilling, followed by a trepanning step that machines the precise contour of the cooling passage. This process is usually fully automated using expensive 5-axis CNC workcells equipped with a powerful Nd-YAG laser.
Problems arise when such a complex component returns after service. Gas turbine blades and vanes are normally designed for an overall lifetime of several 10000 operating hours. The protective coatings, however, are consumed earlier and have to be renewed several times during this period. This is in conflict with the requirement that the original airflow must be maintained.
Many attempts have been made in order to mask cooling holes and to avoid the deposition of coating powder in the cooling channels. Several techniques use masking material either in the form of UV curable material (U.S. Pat. No. 5,726,348, U.S. Pat. No. 6,265,022), epoxy, resin or organic material (EP-A2-1 245 691, EP-A1-1 076 106, U.S. Pat. No. 5,800,695), or fugitive plugs (U.S. Pat. No. 4,743,462). Often the masking material has to be applied in a time consuming manual process, e.g., with a syringe. A suitable etching or heat treatment can subsequently remove the masking material. However, these masking materials do normally not withstand the high temperatures that occur during the plasma deposition of the MCrAlY coating layer, where preheating temperatures exceed 700° C. followed by diffusion bonding heat treatment where temperatures rise above 1000° C. This is also the case for the fugitive plug approach as disclosed in U.S. Pat. No. 4,743,462 where the plug is made from plastic material that volatilizes at a temperature below that of the deposition process.
Due to the considerable number of different exit fan shapes and tolerances in original manufacturing, it is also not practicable to fabricate shadowing masks that could be applied from the outer surface of the part. It is thus desirable to establish a process which does not require any masking and that allows removal of overspray material in a precise and economic way.
The most straightforward method to achieve this goal is manual re-drilling of the partially plugged cooling holes. However, it is usually not easy to identify the precise location of the cooling channels in the oversprayed condition, as a part of the hole may be hidden under the renewed coating. Additionally, even for skilled operators it is not possible to re-contour the fan to the original shape in a reasonable time. As a consequence, the re-drilling operation becomes either prohibitively expensive and time-consuming, or the original cooling effect of the passage is modified in a unacceptable way.
U.S. Pat. No. 5,702,288 offers a solution to this problem. An abrasive slurry is forced through the cooling holes from the inside of the component thus removing residual overspray coating. However, this approach also abrades the other walls of the cooling channels and thus affects the overall performance of the cooling system.
Laser drilling offers an attractive solution for the manufacturing of cooling channels and it is thus evident to use this method for the redrilling process. A number of techniques have been patented for the laser machining of cooling holes in gas turbine components, e.g., U.S. Pat. No. 6,420,677, WO 02/32614, U.S. Pat. No. 6,359,254, U.S. Pat. No. 6,329,632, or U.S. Pat. No. 6,307,175. However, these methods do not provide a suitable solution for the repair process, where the component may have undergone dimensional changes during service. Repair of such parts requires accurately locating the new position of the cooling channel which is usually different from the original location. In addition, only the blocked part of the cooling hole has to be re-machined without causing damage to the back or side walls of the cooling channels. For this purpose the high pulse energies of conventional flash lamp pumped lasers, such as those cited in patents U.S. Pat. No. 6,420,677, WO 02/32614, U.S. Pat. No. 6,359,254, U.S. Pat. No. 6,329,632, or U.S. Pat. No. 6,307,175, are not suitable, because the energy input per pulse is too high and the volume affected by the laser pulse too big. On the other hand, if the pulse energy is reduced by external attenuation the small repetition rates of these conventional drilling lasers no longer allows an economic process.
The opening of substantially blocked cooling holes with an excimer laser operating in the UV has been disclosed in U.S. Pat. No. 5,216,808. The advantage of this type of laser is the high absorption of the UV wavelength in ceramic material such as TBC, which leads to effective material removal. It is claimed that due to the short pulse length and higher photon energy at the UV wavelength TBC is removed athermally by photo-ablation, resulting in negligible heat input into the material. However, this advantage is less pronounced for the metallic MCrAlY coating beneath the TBC layer. Furthermore, solid state lasers such as the Nd-YAG type are generally preferred by the industry for material processing due to their proven reliability and widespread use.
U.S. Pat. No. 6,172,331 and U.S. Pat. No. 6,054,673 give a suitable example of a solid state laser, which is capable of drilling both metallic and ceramic material. Here, a Nd-YAG laser is used in the q-switch mode, where short pulses of less than 500 ns duration are generated. In the q-switched mode the peak pulse power is high enough to remove the material mostly as vapor instead of melt ejection, which is common for conventional drilling. The energy per laser pulse is small and it is thus possible to detect hole breakthrough with a suitable device before significant damage to the wall behind the cooling hole occurs. Although the use of such a laser is desirable, it is not disclosed how the process can be advantageously applied for the repair of components that have undergone dimensional changes or where the cooling hole is only partially obstructed. Furthermore, the focus of these patents is the interruption of the drilling process directly after the detection of breakthrough (U.S. Pat. No. 6,054,673) and the additional use of a frequency multiplied component from the same laser which results in a shorter wavelength and thus higher absorption in metallic and ceramic material (U.S. Pat. No. 6,172,331). It is interesting to note, that U.S. Pat. No. 6,172,331 distinguishes the use of additional harmonic generation from the use of only the original wavelength. A process parameter window covering pulse peak powers from 105 W to 107 W is claimed. However, under certain conditions it is also possible to achieve evaporation of coating material with short pulses at smaller peak power <105 W, which is advantageous, because it reduces the power requirements for the laser source.
The modification and repair of film cooling holes in gas turbine components is described in U.S. Pat. No. 6,243,948, where the cooling hole outlets are enlarged and any portion which might exhibit cracks is removed. Although the inlet of these cooling channels is not modified and thus the total airflow change is very small, the enlargement of the outlets changes the film cooling effect and thus the performance of the component. No details are given about the hole detection or the machining step and how both can be carried out in a precise and economic way.
Such an automated method is suggested in U.S. Pat. No. 6,380,512, where a laser drilling process is disclosed to remove coating material from (partially) blocked cooling passages. The method relies on a 5-axes CNC workcell and a CNC component program with pre-programmed locations of the cooling holes. The drilling apparatus is equipped with a vision system and can thus compensate for component deformation or deviation from blueprint dimensions. The vision equipment is used to determine the actual location of the cooling holes either on the coated component or in a condition where the component is being prepared for coating. The apparatus is equipped with a flash lamp pumped Nd-YAG laser and the vision system is either mounted to the laser such that an image is obtained through the laser lens or it is separated from the laser. With this method it is also possible to remove the component from the fixture that was used for the original drilling, to modify (coat) it thereafter, followed by accurate repositioning and adjustment of the orientation of the component. However, the method only detects the position of the hole and not the orientation of the channel axis. For this reason only a partial compensation of component deformation and manufacturing tolerances is possible with the disclosed technique. Moreover, as the real orientations of the cooling channels are not detected, it is not possible to align the re-machined diffuser outlet accurately with the cylindrical section, which is left from the original drilling.